Device for detecting particulate and one or more gases in the air

ABSTRACT

A MEMS device for detecting particulate and gases in the air, comprising: a first semiconductor body; a second semiconductor body with a first surface facing a first surface of the first semiconductor body; and a first spacer element and a second spacer element, which extend between the first surfaces of the semiconductor bodies so as to arrange them at a distance apart from one another and define a first duct. The MEMS device further comprises at least one of the following: a first particulate sensor comprising a first emitter unit for generating acoustic waves in the first duct, and a first particulate-detection unit for detecting the particulate, the first emitter unit and the first particulate-detection unit facing one another through the first duct; and a first gas sensor, which faces the first duct and is configured to detect said gases in the air present in the first duct.

BACKGROUND Technical Field

The present disclosure relates to a device for detecting particulate andone or more gases in the air. In particular, it regards an integratedMEMS sensor configured to detect gases and particulate, an apparatuscomprising the MEMS device (or sensor), and a method for manufacturingthe MEMS device.

Description of the Related Art

As is known, environmental sensors enable detection of parameters suchas the quality of air, atmospheric pollution, and the presence ofcertain gases in the atmosphere. This is of fundamental importance fordetermining the quality of life in given populated areas and foradopting some precautions or remedies in order to reduce or contain thenumber of elements harmful for the health of human beings (e.g., controlof traffic according to the levels of pollution detected).

In the last few years, various portable apparatuses have been marketed,which comprise one or more environmental sensors inside them and enablethe user (even a non-expert user, e.g., even an average buyer who doesnot use them for professional purposes) to evaluate the quality of theair of the place where he or she is. This should make it possible tomake people aware of the problem of atmospheric pollution, but above allmake it possible for the user to adopt some precautions to counter thepotential harmful effects of pollution (e.g., wearing filtering maskswhen moving around in the town when the levels of pollution exceedcritical thresholds).

In particular, the above portable apparatuses generally enable detectionof atmospheric particulate (with a size usually comprised betweenapproximately 1 μm and approximately 10 μm of diameter) and, in someembodiments, of specific gases harmful for the health of humans (e.g.,CO, CO₂, NO, NO₂).

However, portable apparatuses currently on sale are voluminous, complexto produce, costly and thus not convenient to carry around and use in sofar as they comprise various environmental sensors, each of amacroscopic type, which are each manufactured independently and are thenassembled together on an electronic board (e.g., a printed circuit boardor PCB) and are housed within the portable apparatus. This renderscurrently known portable apparatuses difficult to use bynon-professional users (e.g., by users who do not use them frequently intheir normal working activity).

In addition, environmental sensors require a periodic maintenance thatat the moment calls for a manual intervention by the user (or a periodiccheck by expert professionals, thus causing inconvenience oforganization and additional costs for the user).

BRIEF SUMMARY

The present disclosure provides a MEMS device, an apparatus comprisingthe MEMS device, and a method for manufacturing the MEMS device thatwill, among others, overcome the drawbacks of the existing solutions.

According to the present disclosure a MEMS device, an apparatuscomprising the MEMS device, and a method for manufacturing the MEMSdevice are provided.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, embodimentsthereof are now described with reference to the attached drawings,wherein:

FIG. 1 is a schematic lateral sectional view of a MEMS device fordetecting atmospheric particulate and gases, according to an embodimentof the MEMS device, the MEMS device comprising a particulate sensor anda gas sensor;

FIG. 1A is a schematic illustration of an apparatus comprising the MEMSdevice of FIG. 1 ;

FIGS. 2A and 2B are lateral sectional views of respective details of theparticulate sensor comprised in the MEMS device of FIG. 1 , according toan embodiment;

FIG. 3A is a top view of the gas sensor comprised in the MEMS device ofFIG. 1 , according to an embodiment of the gas sensor;

FIG. 3B is a schematic lateral sectional view of the gas sensor of FIG.3A;

FIGS. 4A and 4B are lateral sectional views of an air-flow sensor insome embodiments comprised in the MEMS device of FIG. 1 , according torespective embodiments;

FIG. 5 is a perspective view of the MEMS device of FIG. 1 , according toan embodiment;

FIGS. 6A and 6B are top views of respective portions of the MEMS deviceof FIG. 5 , rotated with respect to one another;

FIG. 7 is a perspective view of the MEMS device of FIG. 1 , according toan embodiment;

FIGS. 8A and 8B are top views of respective portions of the MEMS deviceof FIG. 7 , rotated with respect to one another;

FIGS. 9A-9I are schematic lateral sectional views that illustraterespective steps for manufacturing the gas sensor and the particulatesensor of the MEMS device of FIG. 5 , according to an embodiment; and

FIGS. 10A-10G are schematic lateral sectional views that illustraterespective steps for manufacturing the MEMS device of FIG. 5 , accordingto an embodiment.

Elements that are in common to the various embodiments of the presentdisclosure, described herein, are designated by the same referencenumbers.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration, in lateral sectional view in a(triaxial) Cartesian reference system of axes X, Y, Z, of a MEMS device20, for example, a MEMS sensor for detecting atmospheric particulate andgases. For example, the MEMS device 20 is configured to be mounted in anapparatus (illustrated in FIG. 1A by the reference number 100, such as asmartphone or personal protective equipment “PPE”) so as to make itpossible for the latter to acquire in a simple and fast way informationon the environment where it is located, for example, information on thequality of air.

The MEMS device 20 comprises a first semiconductor body 22 (ofsemiconductor material such as silicon) and a second semiconductor body23 (of semiconductor material such as silicon) facing one another andcoupled together so as to be arranged at a distance from one another,thus forming a duct 24 (e.g., a micro-channel), through which a fluidicmedium, e.g., air, and thus particulate and the gases carried thereby,can circulate. In detail, the first semiconductor body 22 has a mainextension or dimension by way of example parallel to a plane XY (definedby the axes X and Y) and has a first surface 22 a and a second surface22 b opposite to one another with respect to the axis Z, and the secondsemiconductor body 23 has a main extension or dimension by way ofexample parallel to the plane XY and has a respective first surface 23 aand a respective second surface 23 b opposite to one another withrespect to the axis Z. The first surfaces 22 a and 23 a of thesemiconductor bodies 22 and 23 face one another. The semiconductorbodies 22 and 23 are fixed with respect to one another, as discussedmore fully hereinafter, in such a way that the first surfaces 22 a and23 a are arranged at a distance from one another.

For example, in a way not illustrated in FIG. 1 , spacer elements 26(FIG. 5 ) extend in a direction transverse to the plane XY, and thus ina direction transverse, for example, orthogonal, to the first surfaces22 a and 23 a, to join together the semiconductor bodies 22 and 23keeping them at a distance from one another. According to an embodimentillustrated in FIG. 5 , the MEMS device 20 comprises a first spacerelement 26 a and a second spacer element 26 b, which are fixed to thefirst surfaces 22 a and 23 a and are interposed between thesemiconductor bodies 22 and 23.

As illustrated in FIG. 5 , the semiconductor bodies 22 and 23 and thespacer elements 26 a and 26 b externally define and delimit the duct 24,which has a main extension or dimension direction by way of exampleparallel to the axis X. The duct 24 is in fluidic and pneumaticcommunication with an environment external to the MEMS device 20 througha first opening 24 a and a second opening 24 b opposite to one anotheralong the axis X. In other words, the semiconductor bodies 22 and 23 andthe spacer elements 26 a and 26 b define an internal volume of the MEMSdevice 20 that is delimited along the axes Y and Z and is open towardsthe outside through the first and second openings 24 a and 24 b. Aircoming from the outside can thus traverse the duct 24 entering from thefirst opening 24 a and exiting from the second opening 24 b, or viceversa, as discussed more fully herein.

The MEMS device 20 comprises a particulate sensor 30 for detectingparticulate (designated in FIG. 1 by the reference number 34′) in theair present in the duct 24, and one or more gas sensors 32 for detectingone or more gases (herein reference is made by way of example to a gasdesignated by the reference number 34″, such as NO₂ or NH₃) that may bepresent in the air.

In detail, the particulate sensor 30 comprises an emitter unit 36 thatcan be governed for generating acoustic waves in the duct 24, and aparticulate-detection unit (herein also referred to as PM-detectionunit) 37 for detecting the particulate 34′. The emitter unit 36 and thePM-detection unit 37 are carried by the first semiconductor body 22 andby the second semiconductor body 23 and extend from sides opposite toone another of the duct 24 so as to face each other. Considered by wayof example herein is the case where the emitter unit 36 is comprised inthe first semiconductor body 22 and is formed at the first surface 22 aof the latter and where the PM-detection unit 37 is comprised in thesecond semiconductor body 23 and is formed at the first surface 23 a ofthe latter; however, this case is here considered only by way ofnon-limiting example, and it is thus also possible for the emitter unit36 to be comprised in the second semiconductor body 23 and for thePM-detection unit 37 to be comprised in the first semiconductor body 22.

In detail, the emitter unit 36 and the PM-detection unit 37 arerespective ultrasonic transducer devices, for example, piezoelectricultrasonic transducers obtained with MEMS technology, referred to aspiezoelectric micromachined ultrasonic transducers “PMUTs” ( ).

In use, as described more fully hereinafter, the emitter unit 36generates acoustic waves, designated in FIG. 1 by the reference number48, which propagate in a direction transverse with respect to the duct24, e.g., the direction of propagation is parallel to the axis Z, anddeflect the paths of the particulate 34′ towards the PM-detection unit37, by acoustophoresis. For example, the particulate 34′ present in theduct 24 undergoes, on account of the acoustic waves 48 (which arestanding ultrasonic waves in the duct 24), forces of acoustic radiationthat cause transport of the particulate 34′ towards a nodal plane ofpressure that is located at the PM-detection unit 37. Consequently, onaccount of a pressure caused by the acoustic waves 48 that is lower atthe PM-detection unit 37 as compared to the rest of the duct 24, theparticulate 34′ is forced towards the PM-detection unit 37, to which itthus remains attached as discussed more fully herein.

In use, the PM-detection unit 37 detects the particulate 34′ that hasgathered on the PM-detection unit 37 itself on account of the acousticwaves 48. For example, the PM-detection unit 37 is provided so as topresent frequencies of mechanical vibrations different from thefrequency of the ultrasonic waves generated by the emitter unit 36.Consequently, the PM-detection unit 37 does not vibrate as a result ofthe acoustic waves 48 emitted by the emitter unit 36. The PM-detectionunit 37 is arranged in vibration by an electrical signal, as describedmore fully herein, in order to detect the effective resonance frequencyof the PM-detection unit 37. The mass of the particulate matter 34′ thathas gathered on the PM-detection unit 37 determines a variation of themass of the PM-detection unit 37, and thus a modification of theresonance frequency of the latter. The variation of the resonancefrequency of the PM-detection unit 37 is thus a function of the amountof particulate 34′ present on the PM-detection unit 37. By measuring theresonance frequency of the PM-detection unit 37 it is thus possible toobtain information as regards parameters such as the amount and size ofthe particulate 34′ present in the air.

FIG. 2A shows the structure of the emitter unit 36.

As illustrated in FIG. 2A, the emitter unit 36 is formed in the firstsemiconductor body 22, at the respective first surface 22 a.

A first buried cavity 40 extends in the first semiconductor body 22 atthe first surface 22 a of the first semiconductor body 22, and isseparated from the duct 24 through a membrane 42 suspended over thefirst buried cavity 40. As described more fully herein, the first buriedcavity 40 may be in fluidic connection with the duct 24 through one ormore holes provided in the membrane 42, or may be in fluidic connectionwith the environment external to the MEMS device 20 through one or moreholes or openings in the first semiconductor body 22 that reach thesecond surface 22 b of the first semiconductor body 22, or may beisolated from a fluidic standpoint both with respect to the duct 24 andwith respect to the external environment (in this case, in someembodiments, a predefined amount of air is introduced into the firstburied cavity 40 during manufacture of the emitter unit 36).

For example, the membrane 42 is formed by a membrane body 44 that isprovided starting from the first semiconductor body 22 and that formsthe first surface 22 a of the first semiconductor body 22, and by anactuation unit 46, which is mechanically coupled to the firstsemiconductor body 22 (which, for example, extends over the firstsemiconductor body 22) and can be configured for inducing the membrane42 to vibrate.

In detail, the first semiconductor body 22 comprises a substrate 22′ ofsemiconductor material (e.g., silicon) and an oxide layer 22″, e.g., ofinsulating material such as silicon dioxide, which extends over thesubstrate 22′ and forms the first surface 22 a of the firstsemiconductor body 22. The portion of substrate 22′ and oxide layer 22″suspended over the first buried cavity 40 forms the membrane body 44.

Furthermore, the actuation unit 46 comprises a piezoelectric layer 46 a(of piezoelectric material such as PZT) interposed along the axis Zbetween a first electrode 46 b and a second electrode 46 c (which areconductive, for example of metal material such as platinum, gold, orcopper or of semiconductor material with high concentration of dopantspecies such as silicon with a concentration of dopant species of an Ntype higher than 10¹⁸ at/cm³), which can be biased for generating apotential difference across the piezoelectric layer 46 a and forinducing consequently an elastic deformation of the piezoelectric layer46 a, and thus of the membrane 42, by the reverse piezoelectric effect.For instance, the first electrode 46 b extends in contact with themembrane body 44 (thus on the oxide layer 22″, in contact with the firstsurface 22 a of the first semiconductor body 22), and the secondelectrode 22 b extends on the opposite side of the piezoelectric layer46 a with respect to the first electrode 46 b.

The structure of the PM-detection unit 37, illustrated in FIG. 2B, issimilar to that of the emitter unit 36 and thus is not described again.However, the first buried cavity 40 of the PM-detection unit 37 maycontain air, and thus is in fluidic connection with the duct 24 throughone or more holes in the membrane 42, or is in fluidic connection withthe environment external to the MEMS device 20 through one or more holesor openings in the second semiconductor body 23 that reach the secondsurface 23 b of the second semiconductor body 23, or is isolated from afluidic standpoint both with respect to the duct 24 and with respect tothe external environment but air is introduced into the first buriedcavity 40 during manufacture (e.g., at approximately the same pressurethat the air circulating in the duct 24 will have in use). In someembodiments, in addition to what has been said with reference to theemitter unit 36, the PM-detection unit 37 further comprises, asillustrated in FIG. 2B, a particulate-adhesion layer 49 (herein alsoreferred to as PM-adhesion layer 49, for example of polymeric materialsuch as photoresist), which extends over the membrane 42 (in detail,over the actuation unit 46 and, in some embodiments, on part of thefirst surface 23 a of the second semiconductor body 23) and improvesadhesion of the particulate 34′ to the PM-detection unit 37. Saidadhesion is selective thanks to the choice of the material of thePM-adhesion layer 49 (a choice that is based upon criteria in themselvesknown), and thus only the particulate 34′ adheres and remains bonded tothe emitter unit 36.

In use, the emitter unit 36 faces, and is in contact with, the air inthe duct 24.

In use, the emitter unit 36 is operated in a transmitting mode thereof(e.g., it functions as actuator), and thus the membrane 42 is arrangedin vibration by the actuation unit 46. Vibration of the membrane 42causes generation and propagation in the air of the acoustic waves 48;this is obtained by applying an actuation voltage of an AC type betweenthe electrodes 46 b and 46 c, and thus controlling the mechanicaldeformation of the piezoelectric layer 46 a. In detail, the emitter unit36 is controlled to have the membrane 42 that vibrates to a firstresonance frequency comprised, by way of example, between approximately100 kHz and approximately 1 MHz. The acoustic waves 48 of a stationarytype deflect, by the acoustophoretic effect, the paths of theparticulate 34′, which passes though the duct 24, towards thePM-detection unit 37. For example, the deviation of the paths of theparticulate 34′ increases as the size of the particulate 34′ increases(in other words, the particulate with a greater equivalent diameter isdeflected more towards the PM-detection unit 37).

In use, the PM-detection unit 37 is substantially inert in regard to theacoustic waves 48; in other words, the PM-detection unit 37 is designedto vibrate (also considering the possible contribution provided by theparticulate 34′ that adheres thereto) at second frequencies that aredifferent from the first resonance frequency of the emitter unit 36. Forexample, when the acoustic waves 48, which are generated by the emitterunit 36 and which propagate in the air present in the duct 24, impingeupon the membrane 42 of the PM-detection unit 37, a first part of theenergy of the acoustic waves 48 is reflected towards the emitter unit36, whereas a second part propagates as elastic waves through thePM-detection unit 37 (in detail, in the respective first buried cavity40). Consequently, the emitter unit 36, the duct 24, and thePM-detection unit 37 form a multilayer acoustic resonator. In order todetect the resonance frequency thereof (referred to herein as “secondresonance frequency”), the PM-detection unit 37 is arranged in vibrationin a pre-set range of second frequencies. For example, a chirp signal(e.g., having frequencies varying, in time, between approximately 80 kHzand approximately 120 kHz, and in any case so as to comprise in saidfrequency range the second resonance frequency) is supplied, asdescribed more fully herein, to the PM-detection unit 37 in order tocause vibration of the membrane 42 of the PM-detection unit 37. Thisvibration varies in time on the basis of the chirp signal: when thechirp signal reaches the second resonance frequency (which depends uponthe design of the PM-detection unit 37 and upon the particulate 34′attached thereto, if present), the membrane 42 of the PM-detection unit37 resonates. Consequently, by measuring the frequency response of thevibration of the membrane 42 of the PM-detection unit 37 it is possibleto detect the second resonance frequency. By comparing then the secondresonance frequency measured with a reference resonance frequency (e.g.,the second resonance frequency of the PM-detection unit 37 in theabsence of particulate 34′), it is possible to determine the amount ofparticulate 34′ on the PM-detection unit 37. In greater detail, therelations Δf=f_(meas)−f_(ref), with Δf=k·m_(part)·f_(ref),Δf=k·m_(part)·f_(rif) apply, where f_(meas) f_(mes) is the secondresonance frequency, f_(rif) f_(ref) is the reference resonancefrequency, m_(part) m_(part) is the mass of the particulate 34′ attachedto the PM-detection unit 37, k is a multiplicative constant dependingupon the structure (e.g., geometry and materials) of the PM-detectionunit 37 (e.g., is of the order of magnitude of 10⁻⁵ ng⁻¹ or 10⁻⁵ pg⁻¹),and Δf is the variation of the second resonance frequency caused by theparticulate 34′ on the PM-detection unit 37. In greater detail, thechirp signal causes vibration of the membrane 42 of the PM-detectionunit 37, which generates an elastic deformation of the actuation unit46, which in turn causes generation of a detection current between theelectrodes 46 b and 46 c (in fact, the PM-detection unit 37 can berepresented with the equivalent circuit constituted by a capacitor inparallel with an RLC circuit). By acquiring the detection current, whichis a function of the second resonance frequency, it is thus possible toobtain information on vibration of the membrane 42 of the PM-detectionunit 37. For instance, the reference resonance frequency is comprisedbetween 100 kHz and 1 MHz (in any case, it is different from the firstdetection frequency, and for example a difference between the first andsecond detection frequencies is greater than approximately 50 kHz), andthe second resonance frequency can vary by approximately 1% with respectto the value of the reference resonance frequency.

Further, the PM-detection unit 37 can also function as actuator to favordetachment of the particulate 34′ and thus cleaning of the PM-detectionunit 37. This operating mode of the PM-detection unit 37 is alsoreferred to herein as self-cleaning mode of the PM-detection unit 37 orof the particulate sensor 30. For example, in the self-cleaning mode,the membrane 42 is arranged in vibration by the actuation unit 46 so asto favor detachment of the particulate 34′ from the PM-detection unit37. As already described for the emitter unit 36, vibration of themembrane 42 of the PM-detection unit 37 causes generation andpropagation in air of stationary acoustic waves; this is obtained byapplying a further actuation voltage of an AC type between theelectrodes 46 b and 46 c, and thus controlling mechanical deformation ofthe piezoelectric layer 46 a. In detail, the PM-detection unit 37 iscontrolled, in self-cleaning mode, to present amplitudes of oscillationof the membrane 42 greater than those used for detecting the particulate34′ (e.g., approximately 50 to 100 times greater), and/or by signalswith non-sinusoidal waveforms (e.g., square-wave or triangular-wavesignals). The usefulness of operating the PM-detection unit 37 inself-cleaning mode lies mainly in the fact that, by causing the membrane42 of the PM-detection unit 37 to vibrate, detachment of the particulate34′ previously attached to the latter, and thus cleaning of thePM-detection unit 37, is favored and speeded up. Consequently, byoperating periodically the PM-detection unit 37 in self-cleaning mode(e.g., by operating the PM-detection unit 37 in self-cleaning modewhenever the PM-detection unit 37 has been working for a period longerthan a threshold period in reception mode), it is possible to clean theparticulate 34′ previously attached away from the PM-detection unit 37,thus restoring periodically the original mechanical and electricalproperties of the PM-detection unit 37 and preventing phenomena ofsaturation of the particulate 34′ on the PM-detection unit 37 andconsequent wrong measurements of the particulate 34′ present.

With reference again to FIG. 1 , it is shown that the MEMS device 20comprises said one or more gas sensors 32. FIG. 1 illustrates by way ofexample just one gas sensor 32 provided on the first surface 22 a of thefirst semiconductor body 22. However, it is clear that the number andposition of gas sensors 32 may vary. For instance, in a way notillustrated, the MEMS device 20 may comprise two gas sensors 32 providedon the first surfaces 22 a and 23 a of the first and secondsemiconductor bodies 22 and 23, respectively, so that they face oneanother through the duct 24. Otherwise, the number of gas sensors 32 mayeven be greater than two.

The gas sensor 32 is configured to detect one or more gases 34″ presentin the air that pass through the duct 24. Described by way of exampleherein is the case where the gas sensor 32 is designed for detectingjust one type of gas 34″ (e.g., CO, CO₂, NO, NO₂). However, what isdiscussed herein applies in a similar way to the case where the gassensor 32 is designed for detecting simultaneously different types ofgases 34″.

FIGS. 3A and 3B show the gas sensor 32 according to an embodiment.

For example, the gas sensor 32 illustrated in FIGS. 3A and 3B is formedin the first semiconductor body 22 (comprising the substrate 22′ and theoxide layer 22″), on a second buried cavity 53.

A first heater element 52, e.g., of conductive material, for examplemetal such as platinum or gold, extends over the oxide layer 22″ so asto overlie vertically the second buried cavity 53, and makes itpossible, when traversed by an electric current, to heat the gas sensor32 by the Joule effect. In detail, the first heater element 52 is formedby a metal strip 52′ shaped so as to have a spiral shape, so as tomaximize the path that the electric current has to follow to traversethe first heater element, consequently maximizing the heating suppliedby the first heater element 52. In some embodiments, the first heaterelement 52 further comprises a first heater pad 52″ and a second heaterpad 52″ joined to respective ends of the strip 52′ that are opposite toone another with respect to the strip 52; the heater pads 52″ enableelectrical connection to an interface module 102 of the apparatus 100(the interface module 102 being external to the MEMS device 20 anddesigned to bias the first heater element 52).

An electrically insulating layer 54 (of dielectric material such asSi₃N₄) extends over the first heater element 52 and over the portions ofthe first surface 22 a of the first semiconductor body 22 not covered bythe first heater element 52.

In addition, a gas-detection unit 56 extends on the electricallyinsulating layer 54. The gas-detection unit 56 comprises a first plate56 a and a second plate 56 b capacitively coupled together thanks to theelectrically insulating layer 54; in other words, the plates 56 a and 56b form, with the electrically insulating layer 54, a capacitor. Forinstance, in order to maximize the capacitive coupling between theplates 56 a and 56 b, both the first plate 56 a and the second plate 56b comprise a respective plurality of electrodes, and the electrodes ofthe first plate 56 a are interdigitated with the electrodes of thesecond plate 56 b. Furthermore, the gas-detection unit 56 comprises agas-adhesion layer 58 (e.g., of SnO), which extends over the plates 56 aand 56 b and favors adhesion of the gas 34″ to the gas sensor 32. Saidadhesion is selective thanks to the choice of the material of thegas-adhesion layer 58 (a choice that is based on criteria in themselvesknown), and thus only the gas 34″ to be detected adheres and remainsattached to the gas-detection unit 56. In some embodiments, thegas-detection unit 56 further comprises a first detection pad and asecond detection pad 56″ joined to respective ends of the first andsecond plates 56 a and 56 b, respectively, which are opposite to oneanother with respect to the gas-detection unit 56; the detection pads56″ enable electrical connection with the interface module 102, which isexternal to the MEMS device 20 and is designed to acquire a potentialdifference generated between the first and second plates 56 a and 56 b.

In use, the gas sensor 32 generates said potential difference betweenthe first and second plates 56 a and 56 b, which depends upon the amountof gas 34″ attached to the gas-adhesion layer 58. In fact, adhesion ofthe gas 34″ to the gas-detection unit 56 modifies the electricalproperties of the latter, and thus causes a variation of the voltagebetween the plates 56 a and 56 b with respect to the case where the gas34″ is absent. This variation increases as the amount of gas 34″attached to the gas-adhesion layer 58 increases: consequently, byacquiring the voltage generated between the plates 56 a and 56 b it ispossible to obtain information as regards the amount of gas 34″ in theduct 24.

Further, in use, the first heater element 52 is biased so as to heat itby the Joule effect, thus generating a rise in temperature in the gassensor 32 (in detail, in the electrically insulating layer 54 and in thegas-detection unit 56) due to known effects of heat transfer. Thetemperature of the gas-detection unit 56 affects absorption of the gas34″: it is thus possible to maximize the amount of gas 34″ on thegas-detection unit 56 by heating the latter, by the first heater element52, to a target temperature at which absorption of the gas 34″ ismaximum. This target temperature is specific for each gas and is per seknown.

In addition, it has been found that, by using a same gas-adhesion layer58, it is possible to cause adherence of different types of gases 34″(and thus detect the amount thereof by the gas-detection unit 56) as thetemperature of the gas-detection unit 56 varies. In other words, it isfor example possible to control the first heater element 52 so as tobring the gas-detection unit 56 into a first temperature range at whichthe gas-adhesion layer 58 enables adhesion of a first type of gas 34″and then it is possible to control the first heater element 52differently so as to bring the gas-detection unit 56 into a second rangeof temperatures (e.g., higher than the temperatures of the first range)at which the gas-adhesion layer 58 enables adhesion of a second type ofgas 34″. In this way, the same gas sensor 32 can detect different typesof gases 34″.

With reference once again to FIG. 1 , in some embodiments the MEMSdevice 20 further comprises at least one air-flow sensor (also referredto herein as air sensor) 60, which is configured to measure the air flowthat traverses the duct 24. FIG. 1 illustrates by way of example justone air sensor 60 provided at the first surface 22 a of the firstsemiconductor body 22. However, it is clear that the number and positionof the air sensors 60 can vary. For instance, in a way not illustrated,the MEMS device 20 may comprise two air sensors 60 provided on the firstsurfaces 22 a and 23 a of the first and second semiconductor bodies 22and 23, respectively, so as to face one another through the duct 24.Otherwise, the number of air sensors 60 may also be greater than two.

According to an embodiment illustrated in FIG. 4A, the air sensor 60comprises one or more suspended structures (for example, cantilevers)62. Each suspended structure 62 is bent towards the duct 24, comprises alayer (not shown) of piezoelectric or piezoresistive material and isconfigured to generate an electrical signal (or for modifying its ownelectrical resistance) when the air flow in the duct 24 induces elasticdeformation thereof with respect to an own position of equilibrium. Thisdeformation (and thus the electrical signal generated or the variationof resistance) is proportional to the force applied to the suspendedstructure 62 by the air that traverses the duct 24. For instance, and ina way not illustrated, the air sensor 60 comprises a cavity 64 providedin the first semiconductor body 22 and facing the first surface 22 a(e.g., having a polygonal shape in the plane XY, for example a squareshape), and four cantilevers 62 suspended over said cavity 64 andcoupled to respective lateral surfaces of the cavity 64. Each cantilever62 has a first end 62 a and a second end 62 b opposite to one another,is fixed to the first semiconductor body 22 by its own first end 62 aand has its own second end 62 b free to oscillate. In the restingcondition of the cantilever 62 (e.g., in the absence of forces appliedto the cantilever 62 by the air in the duct 24), the first end 62 a ofthe cantilever 62 is in the plane of the first surface 22 a of the firstsemiconductor body 22, whereas the second end 62 b of the cantilever 62is at a distance, along the axis Z, from the first surface 22 a of thefirst semiconductor body 22 and extends in the duct 24. Consequently,each cantilever 62 protrudes into the duct 24 towards the secondsemiconductor body 23. In use, when an air flow is present in the duct24 (e.g., an air flow having a main direction of propagation parallel tothe axis X), the cantilevers 62 that are opposite to one another withrespect to the cavity 64 undergo forces, generated by the air flow,opposite to one another that cause respective elastic deformationsthereof (one cantilever 62 is deflected towards the first semiconductorbody 22, and the other cantilever 62 is deflected towards the secondsemiconductor body 23) that in turn induce generation of respectiveelectrical signals (in the case where the cantilevers 62 comprise thelayer of piezoelectric material) or of variations of electricalresistance (in the case where the cantilevers 62 comprise the layer ofpiezoresistive material), which can be detected, for example indifferential mode.

According to an embodiment illustrated in FIG. 4B, the air sensor 60comprises a second heater element 67 and two thermopiles 68 facing sidesof the second heater element 67 that are opposite to one another in themain direction of propagation of the air in the duct 24 (here in thedirection of the axis X), and are for example arranged at equaldistances apart from the second heater element 67. For instance, thesecond heater element 67 and the thermopiles 68 are suspended over afurther cavity 66 provided in the first semiconductor body 22 and facingthe first surface 22 a. In use, when no air flow is present in the duct24, the heat generated by the second heater element 67 is detected bythe thermopiles 68 in a symmetrical way (e.g., both of the thermopiles68 detect the same amount of heat generated by the second heater element67); when, instead, an air flow is present in the duct 24, the heatgenerated by the second heater element 67 is detected by the thermopiles68 in an asymmetrical way (e.g., the thermopiles 68 detect amounts ofheat different from one another, since the air flow in the duct 24causes, on account of known thermal effects, a greater transfer of heatto the thermopile 68 downstream of the second heater element 67 in themain direction of propagation of the air as compared to the thermopile68 upstream of the second heater element 67 in the main direction ofpropagation of the air). The thermopiles 68 thus generate respectiveelectrical signals as a function of the heat detected, which areindicative of the air flow in the duct 24.

FIG. 5 shows the MEMS device 20 according to an embodiment thereof. Inaddition, FIGS. 6A and 6B show the first surfaces 22 a and 23 a of thefirst and second semiconductor bodies 22 and 23, respectively, of theMEMS device 20 of FIG. 5 . In the embodiment of FIGS. 5, 6A, 6B, theMEMS device 20 comprises by way of example also an additional gas sensor32 and an additional air sensor 60, which are present in the secondsemiconductor body 23 (however, they are optional) and for example face,respectively, the gas sensor 32 and the air sensor 60 in the firstsemiconductor body 22.

For example, the first surfaces 22 a and 23 a are arranged at a firstdistance d₁ from one another along the axis Z. According to an aspect ofthe present disclosure, the first distance d₁ is approximately equal toone quarter of the wavelength at which the acoustic waves 48 emitted bythe emitter unit propagate (e.g., d₁=λ/4, where λ is the wavelength ofthe acoustic waves 48). This thus enables the emitter unit 36, the duct24, and the PM-detection unit 37 (and thus the corresponding portions ofthe first and second semiconductor bodies 22 and 23) to form theacoustic resonator, as has been said previously. In this way, theacoustic pressure exerted by the acoustic waves 48 in the duct 24 isminimum at the first surface 23 a of the second semiconductor body 23,and this favors aggregation of the particulate 34′ on the PM-detectionunit 37. For instance, the wavelength λ may be equal to approximately3.4 mm, and the first distance d₁ may be equal to approximately 860 μm.

With reference to FIG. 5 , the first and second spacer elements 26 a and26 b form respective side walls of the duct 24, whereas the first andsecond semiconductor bodies 22 and 23 form a bottom wall and a top wall,respectively, of the duct 24.

The particulate sensor 30, the gas sensor 32 and, if present, the airsensor 60 face the duct 24 so as to detect the respective information asregards the air present in the duct 24. According to a non-limitingexample (e.g., where the air enters the duct 24 through the firstopening 24 a and exits from the duct 24 through the second opening 24b), the particulate sensor 30, the air sensor 60 (if present), and thegas sensor 32 extend in succession with respect to one another from thefirst opening 24 a to the second opening 24 b.

In some embodiments, the first surface 22 a of the first semiconductorbody 22 further has first pads 27 electrically coupled in a way notillustrated (e.g., by conductive vias, which extend in the firstsemiconductor body 22 or on the first surface 22 a) to the emitter unit36, the air sensor 60 (if present), and the gas sensor 32 for electricalconnection of these to the interface module 102 external to the MEMSdevice 20, which is configured to bias the emitter unit 36, the airsensor 60 (if present), and the gas sensor 32 and/or to detect thesignals at output therefrom. Furthermore, the first surface 23 a of thesecond semiconductor body 23 further has second pads 29 (FIG. 6B)electrically coupled in a way not illustrated (e.g., by conductive vias,which extend in the second semiconductor body 23 or on the first surface23 a) to the PM-detection unit 37, the air sensor 60 (if present in thesecond semiconductor body 23), and the gas sensor 32 (if present in thesecond semiconductor body 23) for electrical connection of these to theinterface module 102 external to the MEMS device 20, which is configuredto bias the PM-detection unit 37, the air sensor 60, and the gas sensor32 (if present) and/or to detect the signals at output therefrom.

In greater detail, the first pads 27 extend in a region of the firstsurface 22 a of the first semiconductor body 22 that is external to theduct 24 (e.g., it does not face the duct 24) so as to simplifyelectrical contact thereof with the interface module 102. Likewise, thesecond pads 29 extend in a region of the first surface 23 a of thesecond semiconductor body 23 that is external to the duct 24 (e.g., itdoes not face the duct 24) so as to simplify electrical contact thereofwith the interface module 102.

FIG. 7 shows the MEMS device 20 according to an embodiment. Furthermore,FIGS. 8A and 8B show the first surfaces 22 a and 23 a of the first andsecond semiconductor bodies 22 and 23, respectively, of the MEMS device20 of FIG. 7 .

The MEMS device 20 of FIGS. 7, 8A, and 8B is similar to the MEMS deviceof FIG. 5 ; however, three spacer elements 26 are present (the firstspacer element 26 a, the second spacer element 26 b, and a third spacerelement 26 c), which define two ducts 24 (herein referred to as firstand second ducts 24′ and 24″) separated from one another by one of saidspacer elements 26 (e.g., the first spacer element 26 a, which isinterposed along the axis Y between the second and third spacer elements26 b and 26 c). Consequently, the first and second ducts 24′ and 24″ arefluidically in parallel with one another and, more for example, arefluidically isolated from one another (e.g., the two ducts 24′ and 24″do not share the air present in each of them).

In the above embodiment, the particulate sensor 30 extends in the firstduct 24′, whereas the air sensor 60 (if present) and the gas sensor 32extend in the second duct 24″. However, this arrangement is providedpurely by way of non-limiting example: for instance, the air sensor 60can extend in the first duct 24′ instead of in the second duct 24″.

FIG. 1A shows the apparatus 100 comprising the MEMS device 20. Describedby way of example herein is the case where the MEMS device 20 is the oneillustrated in FIG. 5 ; however, this discussion likewise applies alsoto the other embodiments of the MEMS device 20.

For example, the apparatus 100 comprises: the interface module 102electrically coupled to the MEMS device 20 to bias the latter and toacquire the output signals thereof; a control unit 104 operatively(e.g., electrically) coupled to the interface module 103 for control ofthe latter, and thus for control of the MEMS device 20; an (optional)aeration channel 108 coupled to the first and second openings 24 a and24 b of the duct 24 and configured to arrange in fluidic communicationthe MEMS device 20 with an environment external to the apparatus 100;and a pumping module 106 pneumatically coupled to the duct 24 of theMEMS device 20 (for example, extending in the aeration channel 108) andoperatively (e.g., electrically) coupled to the control unit 104 so thatit can be controlled by the latter to cause a flow of air through theaeration channel 108, and thus through the duct 24 of the MEMS device20. For instance, the control unit 104 and the interface module 102 forma control module (not illustrated) of the apparatus 100, which, in use,governs the MEMS device 20 and receives the signals generated at outputby the latter.

In use, the pumping module 106 (which is also obtained with MEMStechnology, and which comprises for example a piezoelectric micro-pumpor a micro-fan) is controlled by the control unit 104 for pumping airthrough the aeration channel 108 (between an inlet opening 108 a and anoutlet opening 108 b of the aeration channel 108, which face the outsideof the apparatus 100), thus generating the flow of air that, startingfrom the environment external to the apparatus 100, traverses the duct24 passing from the first opening 24 a to the second opening 24 b.Consequently, the air that flows in the duct 24 flows therein since itis actively pumped by the pumping module 106.

In addition, the interface module 102 is controlled by the control unit104 to bias the particulate sensor 30 (e.g., to bias the emitter unit 36or, when the particulate sensor 30 is operated in the self-cleaningmode, to bias the PM-detection unit 37), the air sensor 60, and the gassensor 32, and to acquire the electrical signals generated by thePM-detection unit 37, by the air sensor 60, and by the gas sensor 32that represent the quality of air in the duct 24. In greater detail, theinterface module 102 further processes, in a known way, the signalsreceived from the particulate sensor 30, the air sensor 60, and the gassensor 32 (e.g., filtering them in frequency, amplifying them, andconverting them into respective digital signals).

The control unit 104 receives from the interface module 102 the signalscoming from the particulate sensor 30, the air sensor 60, and the gassensor 32 and processes them, in a known way, to obtain parametersindicative of the quality of the air of the environment where theapparatus 100 is located (in detail, the concentration in the air ofparticulate matter and gases of interest). For example, the informationon the air flow obtained through the air sensor 60 makes it possible tocalculate the volume of air sampled that flows in the duct 24, and thushave information on the concentrations in the air of the particulate andof gases of interest (and not only absolute values of particulate andgases of interest).

With reference to FIGS. 9A-9I, a method is now described formanufacturing the emitter unit 36 and the gas sensor 32, shown herein asan example with the embodiment illustrated in FIGS. 3A and 3B, startingfrom a same wafer (designated by 140 in FIG. 9A). For simplicity ofdescription the steps for manufacturing the air sensor 60, which areknown, are not discussed herein. Furthermore, the manufacturing methodpresented here is described with reference to the first semiconductorbody 22 of the MEMS device 20 of FIG. 5 ; however, these manufacturingsteps may likewise be applied for manufacturing the second semiconductorbody 23 of the MEMS device 20 (e.g., for manufacturing the PM-detectionunit 37), as well as for obtaining the MEMS device 20 according to theother embodiments discussed previously.

For example, FIG. 9A shows a first wafer 140 of semiconductor material(for example, silicon), having a top surface 140 a and a bottom surface140 b opposite to one another along the axis Z.

With reference to FIG. 9B, the first and second buried cavities 40 and53 are provided in the first wafer 140, alongside one another and inpositions corresponding to the top surface 140 a of the first wafer 140.

According to an aspect of the present disclosure, provided by way ofnon-limiting example, the buried cavities 40 and 53 are obtained througha Vensens process (also referred to as Venice process). In detail, worktrenches (not illustrated) are formed in regions of the first wafer 140that are to become the buried cavities 40 and 53. Formed in each ofthese regions of the first wafer 140, for example using knownlithographic and selective chemical etching steps, is a set of worktrenches that delimit a respective plurality of pillars (notillustrated) of semiconductor material. By a step of epitaxial growth,an epitaxial layer (not illustrated) is grown on the surface of thefirst wafer 140 (which thus increases in thickness), facing which arethe work trenches. There are then carried out one or more steps ofannealing of the first wafer 140, for example in a reducing environment,for instance in a hydrogen atmosphere, at high temperatures, for examplehigher than 1000° C. Said one or more annealing steps cause a migrationof the semiconductor atoms, here silicon, which tend to move into aposition of lower energy: consequently, the semiconductor atoms of thepillars migrate completely, forming the buried cavities 40 and 53. Theburied cavities 40 and 53 are thus delimited at the top by asemiconductor layer, comprising in part atoms grown epitaxially and inpart migrated atoms, which forms a layer for closing the first wafer140.

With reference to FIG. 9C, in some embodiments, the first wafer 140undergoes an step of thermal oxidation (e.g., of a wet type, carried outat temperatures comprised between 600° C. and 1200° C.) to form theoxide layer 22″ (here of SiO₂, and also referred to herein as top oxidelayer 22″) on the top surface 140 a of the first wafer 140, and a bottomoxide layer 142 (here of SiO₂) on the bottom surface 140 b of the firstwafer 140. For example, the first wafer 140, processed as discussed,forms the substrate 22′ that, together with the top oxide layer 22″,forms the first semiconductor body 22. In addition, the portion of thefirst semiconductor body 22 (e.g., of the substrate 22′ and of the topoxide layer 22″) suspended over the first buried cavity 40 forms themembrane body 44 of the emitter unit 36.

Further, in FIG. 9C, a conductive layer 144 of metal material (e.g.,gold or platinum) is formed on the top oxide layer 22″, for example byphysical deposition (e.g., sputtering or evaporation techniques) orchemical deposition, such as CVD (Chemical Vapor Deposition).

With reference to FIG. 9D, the first electrode 46 b of the actuationunit 46 and the first heater element 52 of the gas sensor 32 are formedstarting from the conductive layer 144. For example, the conductivelayer 144 is processed, for example using known lithographic andselective chemical etching steps, in order to remove selectively firstportions thereof, leaving, instead, second portions thereof (alongsidethe first portions) that form the first electrode 46 b and the firstheater element 52.

Furthermore (FIG. 9D), also formed, in succession with respect to oneanother, are the piezoelectric layer 46 a on the first electrode 46 b(e.g., by deposition using techniques of spin-coating or sputtering withpiezoelectric material, such as PZT), and the second electrode 46 c ofthe actuation unit 46 on the piezoelectric layer 46 a (for instance, byphysical deposition, e.g., with sputtering or evaporation techniques, orchemical deposition, e.g., with CVD, of metal material, such as gold orplatinum). In this way, the actuation unit 46 for operating the emitterunit 36 is formed.

With reference to FIG. 9E, in some embodiments a passivation layer 146is formed (for instance, by deposition, e.g., with sputteringtechniques, of one or more insulating materials, such as Si₃N₄) on theactuation unit 46 and on the first heater element 52. The portion of thepassivation layer 146 that extends over the first heater element 52forms the electrically insulating layer 54 described previously. Forexample, the passivation layer 146 exposes electrical-contact regions ofthe first electrode 46 b, of the second electrode 46 c, and of the firstheater element 52, in order to enable electrical contact thereof withthe outside world (for example, with the interface module 102).

In addition, in some embodiments in FIG. 9E contact elements 148 ofconductive material, e.g., of metal such as Ti, are formed in theseelectrical-contact regions that are exposed by the passivation layer146. The contact elements 148 are electrically coupled, respectively, tothe first electrode 46 b, the second electrode 46 c, and the firstheater element 52. In a way not illustrated or discussed in detail, thecontact elements 148 can be electrically connected to the interfacemodule 102 (e.g., they can be electrically connected by respectiveconductive vias to the first pads 27, which in turn will be electricallyconnected, for example by wire-bonding techniques, to the interfacemodule 102).

Furthermore, in some embodiments a first fluidic-communication hole 150is provided (e.g., by known lithographic and selective-chemical-etchingsteps) through the entire membrane body 44 so as to enable inlet of airinto the first buried cavity 40. However, even though the firstfluidic-communication hole 150 is illustrated by way of example in FIG.9E, it may be absent in the solutions illustrated in FIGS. 9H and 9Idiscussed herein.

With reference to FIG. 9F, the gas-detection unit 56 is provided on theelectrically insulating layer 54. For example, the first and secondplates 56 a and 56 b are formed on the electrically insulating layer 54(for instance, by physical deposition, e.g., with sputtering orevaporation techniques, or chemical deposition, e.g., with CVD, of metalmaterial, such as gold or platinum) and then, the gas-adhesion layer 58is formed (for instance, by deposition of SnO) on the first and secondplates 56 a and 56 b.

With reference to FIG. 9G, the bottom oxide layer 142 is removed, forexample by wet etching.

Further, according to one aspect of the present disclosure, the secondsurface 22 b of the first semiconductor body 22, e.g., the bottomsurface 140 b of the first wafer 140, undergoes etching, e.g., wetetching, in order to remove the semiconductor present under the secondburied cavity 53. In other words, by a back-etching process, a trench isformed that, starting from the second surface 22 b of the firstsemiconductor body 22, reaches the second buried cavity 53.Consequently, the second buried cavity 53 is in fluidic communicationwith the outside of the first semiconductor body 22, and air can enterthe second buried cavity 53. This maximizes thermal dissipation of theheat generated by the gas sensor 32, improving electrical performanceand reliability thereof.

According to a further aspect of the present disclosure illustrated inFIG. 9H, etching, discussed with reference to FIG. 9G, of the secondsurface 22 b of the first semiconductor body 22 is carried out so as toremove both the semiconductor present under the second cavity 53 and thesemiconductor present under the first buried cavity 40. In other words,in addition to what has been discussed with reference to FIG. 9G, afurther trench is formed that, starting from the second surface 22 b ofthe first semiconductor body 22, reaches the first buried cavity 40.Consequently, the first buried cavity 40 is in fluidic communicationwith the outside of the first semiconductor body 22, and the air canenter the first buried cavity 40. In this case, the firstfluidic-communication hole 150 may be absent.

According to an aspect of the present disclosure, illustrated in FIG. 9Iand alternative or in addition to what has been described with referenceto FIG. 9H, etching discussed with reference to FIG. 9G is carried out,and a second fluidic-communication hole 152 is further formed that,starting from the second surface 22 b of the first semiconductor body22, reaches the first buried cavity 40, arranging it in fluidiccommunication with the outside of the first semiconductor body 22 (e.g.,the air can enter the first buried cavity 40). In other words, inaddition to what has been discussed with reference to FIG. 9G there isformed (e.g., by known lithographic and selective-chemical-etchingsteps) the second fluidic-communication hole 152, which, starting fromthe second surface 22 b of the first semiconductor body 22, traversesthe substrate 22′ until it reaches the first buried cavity 40. Inaddition, the second fluidic-communication hole 152 does not generally(even though it may) overlie vertically (e.g., along the axis Z) themembrane 42 of the emitter unit 36; otherwise, the secondfluidic-communication hole 152 never vertically overlies the membrane 42of the PM-detection unit 37. When the second fluidic-communication hole152 is present, the first fluidic-communication hole 150 may be absent(as illustrated by way of example in FIG. 9I).

The manufacturing method described with reference to FIGS. 9A-9I makesit possible to provide the emitter unit 36 and the gas sensor 32.

In a similar way, it is possible to provide the PM-detection unit 37and, possibly, the further gas sensor 32 in the second semiconductorbody 23. For example, in order to provide the PM-detection unit 37, inaddition to what has been described previously there may further beformed the PM-adhesion layer 49 (when envisaged) on the actuation unit46. In detail, the PM-adhesion layer 49 is provided on the passivationlayer 146, for example by deposition (e.g., spin coating) of polymericmaterial such as photoresist.

According to an embodiment of the manufacturing method (notillustrated), the emitter unit 36 and the gas sensor 32 are obtainedwith SOI (Silicon-On-Insulator) technology.

For example, a first oxide layer is formed (e.g., by thermal oxidation)on a first surface of a first wafer of semiconductor material (silicon).An etch is carried out on the first wafer, in a first region and asecond region of the first oxide layer, in order to form a first trenchand, respectively, a second trench (which are to become the first andsecond buried cavities 40 and 53) that traverse the first oxide layerand extend in part in the first wafer. A second oxide layer is formed(e.g., by thermal oxidation) on a first surface of a second wafer ofsemiconductor material (silicon), which has a second surface opposite tothe first surface. The first and second wafers are coupled together (forexample, bonded together, for example using wafer-bonding techniques) toform a single semiconductor body in such a way that the first and secondoxide layers are in contact with one another. This makes it possible toform the first and second buried cavities 40 and 53. In detail, bondingis carried out approximately in ambient atmosphere (e.g., not in vacuumconditions) so that air will remain trapped in the first and secondburied cavities 40 and 53. There then follow steps similar to the onesdescribed previously to form the emitter unit 36 and the gas sensor 32on the second surface of the second wafer, and in greater detail inpositions corresponding to the first buried cavity 40 and the secondburied cavity 53, respectively.

FIGS. 10A-10G show the process for manufacturing the MEMS device 20.Described herein is the process for manufacturing the MEMS device 20illustrated in FIG. 5 ; however, the following steps apply in a similarway also to the case of the other embodiments of the MEMS device 20described previously (e.g., the MEMS device 20 of FIG. 7 ).

FIG. 10A shows a spacing wafer 160 of semiconductor material (e.g.,silicon) having a first surface 160 a and a second surface 160 bopposite to one another along the axis Z.

With reference to FIG. 10B, a first oxide layer 162 (e.g., of SiO₂) isformed (e.g., by thermal oxidation) on the first surface 160 a of thespacing wafer 160, and a second oxide layer 164 (e.g., of SiO₂) isformed (e.g., by thermal oxidation) on the second surface 160 b of thespacing wafer 160.

With reference to FIG. 10C, a transport wafer 166 of semiconductormaterial (e.g., silicon) is temporarily coupled to the spacing wafer 160by wafer-bonding techniques. For example, the transport wafer 166 has afirst surface 166 a and a second surface 166 b opposite to one anotheralong the axis Z, and is bonded to the first oxide layer 162 by atemporary bonding layer 168 (e.g., of photoresist deposited by spincoating) that extends in contact with the first surface 166 a of thetransport wafer 166.

With reference to FIG. 10D, the second oxide layer 164 is removed (e.g.,by selective etching) and an etch (e.g., of a wet type) is carried outin a first region of the second surface 160 b of the spacing wafer 160so as to form a duct cavity (which is to form the duct 24, and is thusdesignated by the same reference number) that extends right through thespacing wafer 160 until it reaches the first oxide layer 162. Secondregions of the second surface 160 b of the spacing wafer 160, alongsidethe first region of the second surface 160 b of the spacing wafer 160,are not exposed to etching. Consequently, the spacing wafer 160 is notetched in said second regions, and said non-etched portions of thespacing wafer 160 form the respective spacer elements 26 (in detail, thefirst and second spacer elements 26 a and 26 b), which are thusseparated from one another by the duct cavity 24. In detail, each of thespacer elements 26 has a respective first surface (corresponding to partof the first surface 160 a of the spacing wafer 160, and thus designatedby the same reference number) and a respective second surface(corresponding to part of the second surface 160 b of the spacing wafer160, and thus designated by the same reference number) opposite to oneanother along the axis Z, the first oxide layer 162 extending over thefirst surfaces 160 a of the spacer elements 26.

With reference to FIG. 10E, the first semiconductor body 22 is coupledto the spacer elements 26 by wafer-bonding techniques. In detail, thefirst semiconductor body 22 (comprising the emitter unit 36, the gassensor 32, and in some embodiments the air sensor 60, obtained followingthe manufacturing steps discussed with reference to FIGS. 9A-9I) isbonded, on its own first surface 22 a, to the second surfaces 160 b ofthe spacer elements 26. Said bonding is obtained, for example, by afirst bonding layer 170 (e.g., of photoresist such as SU-8), whichextends between the second surfaces 160 b of the spacer elements 26 andthe first surface 22 a of the first semiconductor body 22.

With reference to FIG. 10F, the transport wafer 166 is removed. Indetail, the temporary bonding between the transport wafer 166 and thefirst oxide layer 162 is dissolved, for example by removing (e.g., byetching or heating) the temporary bonding layer 168, thus causing mutualdetachment of the transport wafer 166 from the spacer elements 26.Further, the first oxide layer 162 is removed, for example by etching(e.g., wet etching) so as to expose the first surfaces 160 a of thespacer elements 26.

With reference to FIG. 10G, the second semiconductor body 23 is coupledto the spacer elements 26, on opposite sides of the spacer elements 26with respect to the first semiconductor body 22, by wafer-bondingtechniques. In detail, the second semiconductor body 23 (comprising thePM-detection unit 36 and, in some embodiments, the gas sensor 32 and/orthe air sensor 60, obtained with the manufacturing steps discussedpreviously) is bonded, on its own first surface 23 a, to the firstsurfaces 160 a of the spacer elements 26. This bonding is obtained, forexample, by a second bonding layer 172 (e.g., of photoresist, such asSU-8) that extends between the first surfaces 160 a of the spacerelements 26 and the first surface 23 a of the second semiconductor body23.

In this way, the duct 24 and, more in general, the MEMS device 20 areformed.

From an examination of the characteristics of the disclosure providedaccording to the present disclosure, the advantages that it affords areevident.

The MEMS device 20 is an integrated device, obtained with MEMStechnology, which has reduced dimensions and reduced levels of energyconsumption, and renders possible simultaneous detection of a number ofparameters indicative of the quality of the air (for example, of theconcentration of particulate 34′ and one or more gases 34″ to bemonitored in the air).

Consequently, the MEMS device 20 presents a low manufacturing cost andcan be easily inserted in various apparatuses 100, and this enables agreater diffusion thereof as compared to known devices and apparatuses.In addition, use thereof is simple and maintenance thereof is automatic(self-cleaning mode of the particulate sensor 30).

Consequently, any user can purchase it and use it (also for amateurpurposes), and this promotes a greater awareness in regard to problemsof atmospheric pollution and makes it possible to adopt in a simpler wayand on a large scale given precautions (e.g., wearing filtering masks)when the air does not meet given criteria.

In addition, the sensors discussed previously can be easily integrated,present a high sensitivity, and are not dependent upon variable workingconditions (e.g., humidity, etc.).

Furthermore, the possibility of operating the PM-detection unit 37 inself-cleaning mode guarantees proper operation thereof.

The optional presence of the PM-adhesion layer 49 in the PM-detectionunit 37 further improves adhesion of the particulate 34′ to thePM-detection unit 37, even though it complicates cleaning thereof andcan reduce the total service life during which the PM-detection unit 37can be used.

Finally, it is clear that modifications and variations may be made tothe disclosure described and illustrated herein, without therebydeparting from the scope of the present disclosure, as defined in theannexed claims.

For example, the number and position in the MEMS device 20, of theparticulate sensors 30, the gas sensors 32, and, if present, the airsensors 60 can vary with respect to what has been described. Forinstance, it is possible to have a plurality of particulate sensors 30,a plurality of gas sensors 32, and a plurality of air sensors 60.Furthermore, the gas sensors 32 and the air sensors 60 can be carried bythe second semiconductor body 23 (or in part by the first semiconductorbody 22 and in part by the second semiconductor body 23). For example,having a plurality of particulate sensors 30 arranged in series with oneanother along the axis X makes it possible to discriminate theparticulate matter 34′ also by size. In fact, since, as describedpreviously, the particulate matter 34′ is deflected more byacoustophoresis as its size increases, having this plurality ofparticulate sensors 30 makes it possible to detect the particulatematter 34′ of larger size with the particulate sensors 30 closer to thefirst opening 24 a and to detect the particulate matter 34′ of smallersize with the particulate sensors 30 closer to the second opening 24 b.

For instance, the particulate sensors 30 can be arranged to formone-dimensional or two-dimensional arrays. A similar considerationapplies to the air sensors 60 and the gas sensors 32. Further, the gassensors 32 can be designed (e.g., by varying the gas-adhesion layer 58)for detecting types of gases 34″ different from one another, thusenabling simultaneous measurement of a number of types of gases 34″.

In addition, as illustrated in FIG. 7 , the number of ducts 24 parallelto one another and not communicating may be greater than two.

Furthermore, in order to obtain the first and second buried cavities 40and 53, it is possible to replace the step described in FIG. 9B with arepetition of steps in succession to one another, each step comprisingformation of a layer of semiconductor material (e.g., by epitaxialgrowth of silicon) on the substrate 22′ (which thus increases inthickness at each step), followed by a respective etch for removing twoportions of said layer of semiconductor material. By removing theportions of each layer always in the same regions, it is possible toform two respective trenches, which, once covered by a final layer ofsemiconductor material (e.g., formed by epitaxial growth), define thefirst and second buried cavities 40 and 53.

A MEMS device (20) for detecting particulate (34′) and one or more gases(34″) in the air, may be summarized as including a first semiconductorbody (22) having a first surface (22 a); a second semiconductor body(23) having a respective first surface (23 a) facing the first surface(22 a) of the first semiconductor body (22); and a first spacer element(26 a) and a second spacer element (26 b), which extend between thefirst surfaces (22 a, 23 a) of the first and second semiconductor bodies(22, 23) so as to arrange the first and second semiconductor bodies (22,23) at a distance apart from one another, wherein the first and secondsemiconductor bodies (22, 23) and the first and second spacer elements(26 a, 26 b) form walls of a first duct (24) that has a main extensiondirection (X) and that has a first opening (24 a) and a second opening(24 b) opposite to one another in the main extension direction (X), theMEMS device (20) further comprising at least one of the following afirst particulate sensor (30) including a first emitter unit (36)configured to generate acoustic waves (48) in the first duct (24), and afirst particulate-detection unit (37) configured to detect theparticulate (34′) present in the first duct (24), wherein the firstemitter unit (36) is carried by the first semiconductor body (22) andthe first particulate-detection unit (37) is carried by the secondsemiconductor body (23) in such a way that the first emitter unit (36)and the first particulate-detection unit (37) face one another throughthe first duct (24), transversally with respect to the main extensiondirection (X); and a first gas sensor (32), which is carried by thefirst or second semiconductor body (22, 23), faces the first duct (24)and is configured to detect said one or more gases (34″) in the airpresent in the first duct (24).

Each one between the first emitter unit (36) and the firstparticulate-detection unit (37) may include a respective membrane (42)suspended over a respective first buried cavity (40) realized in thefirst semiconductor body (22) and in the second semiconductor body (23),respectively, at the respective first surfaces (22 a, 23 a), eachmembrane (42) may include a respective actuation unit (46), wherein theactuation unit (46) of the first emitter unit (36) may be configured tocause the vibration, piezoelectrically, of the membrane (42) of thefirst emitter unit (36) so as to cause generation of the acoustic waves(48) in the air present in the first duct (24), and the firstparticulate-detection unit (37) may be configured to detect, by therespective actuation unit (46), a variation of vibration of the membrane(42) of the first particulate-detection unit (37), which depends uponthe particulate (34′) attached to the first particulate-detection unit(37).

The membrane (42) of the first particulate-detection unit (37) mayfurther include a particulate-adhesion layer (49) that extends over therespective actuation unit (46), faces the first duct (24) and isconfigured to cause adherence of the particulate (34′) to said membrane(42).

Each actuation unit (46) may include a piezoelectric layer (46 a)interposed between a first electrode (46 b) and a second electrode (46c), the first electrode (46 b) extending between the piezoelectric layer(46 a) and a respective membrane body (44) of the respective membrane(42), over which the actuation unit (46) extends.

The actuation unit (46) of the first particulate-detection unit (37) maybe further configured to cause the vibration, in a self-cleaning mode ofthe first particulate-detection unit (37), the membrane (42) of thefirst particulate-detection unit (37) so as to cause detachment of theparticulate (34′) present on the membrane (42) of the firstparticulate-detection unit (37) from the membrane (42) of the firstparticulate-detection unit (37).

The first gas sensor (32) may include a first heater element (52), whichextends above a second buried cavity (53) realized in the firstsemiconductor body (22) or in the second semiconductor body (23) at therespective first surface (22 a, 23 a), and may be configured to generateheat when biased; an electrically insulating layer (54), extending onthe first heater element (52); and a gas-detection unit (56) configuredto detect said one or more gases (34″), wherein the gas-detection unit(56) may include a first plate (56 a) and a second plate (56 b), whichmay be configured to be capacitively coupled together and extend overthe electrically insulating layer (54), and a gas-adhesion layer (58),which extends over the first and second plates (56 a, 56 b), faces thefirst duct (24), and may be configured to cause adherence of said one ormore gases (34″) to the gas-detection unit (56).

The MEMS device may further include one or more air-flow sensors (60),which are carried by the first semiconductor body (22) and/or by thesecond semiconductor body (23), face the first duct (24), and areconfigured to measure a flow of air through the first duct (24).

Each air-flow sensor (60) may include one or more suspended structures(62), which extend over a cavity (64) realized in the firstsemiconductor body (22) or in the second semiconductor body (23) andfacing the first duct (24), each suspended structure (62) having an end(62 b) that may be free to oscillate and protrudes into the first duct(24) in such a way that the air flow, when present in the first duct(24), causes elastic deformation of the suspended structure (62).

Each air-flow sensor (60) may include a second heater element (67),which may be suspended over a cavity (66) realized in the firstsemiconductor body (22) or in the second semiconductor body (23) andfacing the first duct (24), and may be configured to generate heat; and

a first thermopile (68) and a second thermopile (68), which facerespective sides of the second heater element (67) that may be oppositeto one another in the main extension direction (X), the first and secondthermopiles (68) being configured to detect the heat generated by thesecond heater element (67).

The MEMS device may further include at least one second particulatesensor (30) including a second emitter unit (36) configured to generatefurther acoustic waves (48) in the first duct (24), and a secondparticulate-detection unit (37) configured to detect the particulate(34′) present in the first duct (24), wherein the second emitter unit(36) is carried by one between the first and second semiconductor bodies(22, 23) and the second particulate-detection unit (37) is carried bythe other one between the first and second semiconductor bodies (22, 23)in such a way that the second emitter unit (36) and the secondparticulate-detection unit (37) face one another through the first duct(24), transversally with respect to the main extension direction (X).

The MEMS device may further include at least one second gas sensor (32),which is carried by the first semiconductor body (22) or the secondsemiconductor body (23), faces the first duct (24), and is configured todetect said one or more gases (34″) in the air present in the first duct(24).

The MEMS device (20) may include the first particulate sensor (30) andthe first gas sensor (32), which both face the first duct (24).

The MEMS device may further include at least one third spacer element(26 c), which extends between the first surfaces (22 a, 23 a) of thefirst and second semiconductor bodies (22, 23) so as to arrange thefirst and second semiconductor bodies (22, 23) at a distance apart fromone another, wherein the first and second semiconductor bodies (22, 23)and the first and third spacer elements (26 a, 26 c) form respectivewalls of a second duct (24) that is fluidically parallel to the firstduct (24), has a respective main extension direction (X) and has arespective first opening (24 a) and a respective second opening (24 b)opposite to one another in the respective main extension direction (X),the MEMS device (20) may further include at least one of the following:at least one third particulate sensor (30), each third particulatesensor (30) including a third emitter unit (36) configured to generaterespective acoustic waves (48) in the second duct (24), and a thirdparticulate-detection unit (37) configured to detect the particulate(34′) present in the second duct (24), wherein the third emitter unit(36) is carried by one between the first and second semiconductor bodies(22, 23), and the third particulate-detection unit (37) is carried bythe other one between the first and second semiconductor bodies (22, 23)in such a way that the third emitter unit (36) and the thirdparticulate-detection unit (37) face one another through the second duct(24), transversally with respect to the respective main extensiondirection (X); and at least one third gas sensor (32), each third gassensor (32) being carried by the first semiconductor body (22) or thesecond semiconductor body (23), facing the second duct (24), and beingconfigured to detect said one or more gases (34″) in the air present inthe second duct (24).

An apparatus (100) may be summarized as including a MEMS device (20).

The apparatus may further include a pumping module (106) of a MEMS typepneumatically coupled to the first duct (24) and configured to generatea flow of air through the first duct (24); and a control module (104,102) configured to command the MEMS device (20) and receive signalstherefrom.

A method for manufacturing a MEMS device (20) for detecting particulate(34′) and one or more gases (34″) in the air, may be summarized asincluding the steps of forming a first spacer element (26 a) and asecond spacer element (26 b) between respective first surfaces (22 a, 23a), facing one another, of a first semiconductor body (22) and of asecond semiconductor body (23) so as to arrange the first and secondsemiconductor bodies (22, 23) at a distance apart from one another,wherein the first and second semiconductor bodies (22, 23) and the firstand second spacer elements (26 a, 26 b) form walls of a first duct (24),which has a main extension direction (X) and has a first opening (24 a)and a second opening (24 b) opposite to one another in the mainextension direction (X); and further forming at least one of thefollowing: a first particulate sensor (30) including a first emitterunit (36) configured to generate acoustic waves (48) in the first duct(24) and a first particulate-detection unit (37) configured to detectthe particulate (34′) present in the first duct (24), wherein the firstemitter unit (36) is carried by the first semiconductor body (22) andthe first particulate-detection unit (37) is carried by the secondsemiconductor body (23) in such a way that the first emitter unit (36)and the first particulate-detection unit (37) face one another throughthe first duct (24) transversally with respect to the main extensiondirection (X); and a first gas sensor (32), which is carried by thefirst semiconductor body (22) or by the second semiconductor body (23),faces the first duct (24), and is configured to detect said one or moregases (34″) in the air present in the first duct (24).

The step of forming the first and second spacer elements (26 a, 26 b)may include fixing a spacing wafer (160) to a transport wafer (166), thespacing wafer (160) having a respective first surface (160 a) and arespective second surface (160 b), and the transport wafer (166) havinga respective first surface (166 a) and a respective second surface (166b), the first surfaces (160 a, 166 a) of the spacing wafer (160) and ofthe transport wafer (166) facing one another; forming in the spacingwafer (160), by etching of the second surface (160 b) of the spacingwafer (160), a duct cavity (24), which may extend from the secondsurface (160 b) of the spacing wafer (160) to the first surface (160 a)of the spacing wafer (160) and may define and separate the first spacerelement (26 a) and the second spacer element (26 b) from one another;fixing the first semiconductor body (22) to the spacing wafer (160) insuch a way that the first surface (22 a) of the first semiconductor body(22) may face the second surface (160 b) of the spacing wafer (160);uncoupling the spacing wafer (160) and the transport wafer (166) fromone another; and fixing the second semiconductor body (23) to thespacing wafer (160) in such a way that the first surface (23 a) of thesecond semiconductor body (23) may face the first surface (160 a) of thespacing wafer (160), thus forming the first duct (24).

The step of forming the first particulate sensor (30) may includeforming, at a first surface (140 a) of a wafer (140), a first buriedcavity (40), said wafer (140) forming the first semiconductor body (22)or the second semiconductor body (23); and forming, on the first surface(140 a) of the wafer (140), an actuation unit (46) of the first emitterunit (36) or of the first particulate-detection unit (37), which mayoverly the first buried cavity (40).

The step of forming the actuation unit (46) may include forming, on thefirst surface (140 a) of the wafer (140), a first electrode (46 b)overlying the first buried cavity (40); forming a piezoelectric layer(46 a) on the first electrode (46 b); and forming a second electrode (46c) on the piezoelectric layer (46 a).

The step of forming the first particulate sensor (30) may furtherinclude forming, on the actuation unit (46) of the firstparticulate-detection unit (37), a particulate-adhesion layer (49)configured to cause adherence of the particulate (34′) to the firstparticulate-detection unit (37).

The step of forming the first gas sensor (32) may include forming asecond buried cavity (53) at the first surface (140 a) of the wafer(140) and alongside the first buried cavity (40); forming, on the firstsurface (140 a) of the wafer (140), a first heater element (52)overlying the second buried cavity (53) and configured to generate heatwhen biased; forming an electrically insulating layer (54) on the firstheater element (52); and forming, on the electrically insulating layer(54), a gas-detection unit (56) configured to detect said one or moregases (34″), wherein the gas-detection unit (56) may include a firstplate (56 a) and a second plate (56 b), which may be configured to becapacitively coupled together and extend over the electricallyinsulating layer (54), and a gas-adhesion layer (58), which may extendover the first and second plates (56 a, 56 b), may face the first duct(24), and may be configured to cause adherence of said one or more gases(34″) to the gas-detection unit (56).

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary to employ concepts of the various embodiments to provide yetfurther embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A MEMS device, comprising: a first semiconductor body having a firstsurface of the first semiconductor body; a second semiconductor bodyhaving a first surface of the second semiconductor body facing the firstsurface of the first semiconductor body; a first spacer element and asecond spacer element, each between the first surface of the firstsemiconductor body and the first surface of the second semiconductorbody, the first and second semiconductor bodies at a distance apart fromone another; a first duct between the first and second semiconductorbodies and between the first and second spacer elements, the first ducthaving a main extension direction and a first opening and a secondopening opposite to one another in the main extension direction; a firstparticulate sensor including a first emitter unit configured to generateacoustic waves in the first duct, and a first particulate-detection unitconfigured to detect a particulate present in the first duct, the firstemitter unit on the first semiconductor body and the firstparticulate-detection unit on the second semiconductor body, the firstemitter unit and the first particulate-detection unit facing one anotherthrough the first duct in a direction that transvers the main extensiondirection; and a first gas sensor on one of the first semiconductor bodyor the second semiconductor body, facing the first duct and configuredto detect a gas in air present in the first duct.
 2. The MEMS deviceaccording to claim 1, wherein the first emitter unit and the firstparticulate-detection unit each comprises a respective membranesuspended over a respective buried cavity in the first semiconductorbody and in the second semiconductor body, respectively, at therespective first surfaces, each membrane comprising a respectiveactuation unit, wherein the actuation unit of the first emitter unit isconfigured to cause the membrane of the first emitter unit vibratepiezoelectrically to generate acoustic waves in a fluidic medium presentin the first duct, and wherein the actuation unit of the firstparticulate-detection unit is configured to detect a variation ofvibration of the membrane of the first particulate-detection unit. 3.The MEMS device according to claim 2, wherein the membrane of the firstparticulate-detection unit further comprises a particulate-adhesionlayer that extends over the actuation unit of the firstparticulate-detection unit, the particulate-adhesion layer facing thefirst duct and configured to cause adherence of the particulate to themembrane of the first particulate-detection unit.
 4. The MEMS deviceaccording to claim 2, wherein each actuation unit comprises a firstelectrode, a second electrode, and a piezoelectric layer interposedbetween the first electrode and the second electrode, the firstelectrode extending between the piezoelectric layer and a respectivemembrane.
 5. The MEMS device according to claim 2, wherein the actuationunit of the first particulate-detection unit is further configured tocause, in a self-cleaning mode of the first particulate-detection unit,the membrane of the first particulate-detection unit to vibrate.
 6. TheMEMS device according to claim 1, wherein the first gas sensorcomprises: a first heater element, which extends over a second buriedcavity in the one of the first semiconductor body or the secondsemiconductor body at the respective first surface, and is configured togenerate heat when biased; an electrically insulating layer, extendingon the first heater element; and a gas-detection unit including: a firstplate and a second plate extending over the electrically insulatinglayer, the first plate and the second plate configured to becapacitively coupled together; and a gas-adhesion layer extending overthe first and second plates, facing the first duct, and configured toadhere the gas.
 7. The MEMS device according to claim 1, furthercomprising one or more air-flow sensors on one of more of the firstsemiconductor body or the second semiconductor body, the one or moreair-flow sensors each facing the first duct, and configured to measure aflow of air through the first duct.
 8. The MEMS device according toclaim 7, wherein the one or more air-flow sensors each comprises one ormore suspended structures, which extend over a cavity in the firstsemiconductor body or in the second semiconductor body and face thefirst duct, each suspended structure having an end that is free tooscillate, protrudes into the first duct, and is configured toelastically deform in response to the flow of air in the first duct. 9.The MEMS device according to claim 7, wherein the one or more air-flowsensor each comprises: a second heater element suspended over a cavityin the first semiconductor body or in the second semiconductor body andfacing the first duct, and configured to generate heat; and a firstthermopile and a second thermopile, which face respective sides of thesecond heater element that are opposite to one another in the mainextension direction, the first and second thermopiles being configuredto detect the heat generated by the second heater element.
 10. The MEMSdevice according to claim 1, further comprising at least one secondparticulate sensor comprising a second emitter unit configured togenerate further acoustic waves in the first duct, and a secondparticulate-detection unit configured to detect the particulate presentin the first duct, wherein the second emitter unit is on one of thefirst and second semiconductor bodies and the secondparticulate-detection unit is on another one of the first and secondsemiconductor bodies, the second emitter unit and the secondparticulate-detection unit facing one another through the first duct inthe direction that transverses the main extension direction.
 11. TheMEMS device according to claim 1, further comprising at least one secondgas sensor on the first semiconductor body or the second semiconductorbody, facing the first duct, and configured to detect the gas in the airpresent in the first duct.
 12. The MEMS device according to claim 1,wherein the MEMS device comprises the first particulate sensor and thefirst gas sensor, which both face the first duct.
 13. The MEMS deviceaccording to claim 1, further comprising at least one third spacerelement, which extends between the first surfaces of the first andsecond semiconductor bodies, wherein the first and second semiconductorbodies and the first and third spacer elements form respective walls ofa second duct that is fluidically parallel to the first duct and has arespective first opening and a respective second opening opposite to oneanother in the main extension direction, the MEMS device furthercomprising at least one of the following: at least one third particulatesensor, each third particulate sensor comprising a third emitter unitconfigured to generate respective acoustic waves in the second duct, anda third particulate-detection unit configured to detect the particulatepresent in the second duct, wherein the third emitter unit is con one ofthe first and second semiconductor bodies, and the thirdparticulate-detection unit is on another one of the first and secondsemiconductor bodies, the third emitter unit and the thirdparticulate-detection unit facing one another through the second duct,in a direction that transverses the main extension direction; and atleast one third gas sensor, each third gas sensor on the firstsemiconductor body or the second semiconductor body, facing the secondduct, and being configured to detect the gas in the air present in thesecond duct.
 14. An apparatus comprising a MEMS device, the MEMS deviceincluding: a first semiconductor body having a first surface; a secondsemiconductor body having a second surface facing the first surface ofthe first semiconductor body; a first spacer element and a second spacerelement between the first surface of the first semiconductor body andthe second surface of the second semiconductor body; a first ductbetween the first and second semiconductor bodies and between the firstand second spacer elements; a first emitter unit on the firstsemiconductor body, the first emitter unit including a first membranesuspended over a first buried cavity in the first semiconductor body;and a first particulate detection unit on the second semiconductor body,the first particulate detection unit including a second membranesuspended over a second buried cavity in the second semiconductor body,the first membrane facing the second membrane through the first duct.15. The apparatus according to claim 14, further comprising: a pumpingmodule of a MEMS type pneumatically coupled to the first duct andconfigured to generate a flow of air through the first duct; and acontrol module configured to command the MEMS device and receive signalsfrom the MEMS device.
 16. A method for manufacturing a MEMS device,comprising: forming a first spacer element and a second spacer elementbetween respective first surfaces of a first semiconductor body and of asecond semiconductor body, wherein the first and second semiconductorbodies and the first and second spacer elements form walls of a firstduct, which has a main extension direction and has a first opening and asecond opening opposite to one another in the main extension direction;and forming at least one of: a first particulate sensor including afirst emitter unit configured to generate an acoustic wave in the firstduct and a first particulate-detection unit configured to detect aparticulate present in the first duct, wherein the first emitter unit ison the first semiconductor body and the first particulate-detection unitis on the second semiconductor body, the first emitter unit and thefirst particulate-detection unit facing one another through the firstduct; and a first gas sensor, on one of the first semiconductor body orthe second semiconductor body, facing the first duct, and configured todetect a gas in air present in the first duct.
 17. The method accordingto claim 16, wherein the forming the first and second spacer elementscomprises: fixing a spacing wafer to a transport wafer, the spacingwafer having a first surface and a second surface, and the transportwafer having a first surface and a second surface, the first surface ofthe spacing wafer and the first surface of the transport wafer facingone another; forming in the spacing wafer a duct cavity, which extendsfrom a second surface of the spacing wafer to the first surface of thespacing wafer, the duct cavity separating the first spacer element andthe second spacer element from one another; fixing the firstsemiconductor body to the spacing wafer, the first surface of the firstsemiconductor body facing the second surface of the spacing wafer;uncoupling the spacing wafer and the transport wafer from one another;and fixing the second semiconductor body to the spacing wafer, the firstsurface of the second semiconductor body facing the first surface of thespacing wafer.
 18. The method according to claim 16, comprising formingthe first particulate sensor, which includes: forming, at a firstsurface of a wafer, a first buried cavity; and forming, on the firstsurface of the wafer, an actuation unit overlapping the first buriedcavity.
 19. The method according to claim 18, wherein the forming theactuation unit comprises: forming, on the first surface of the wafer, afirst electrode overlapping the first buried cavity; forming apiezoelectric layer on the first electrode; and forming a secondelectrode on the piezoelectric layer.
 20. The method according to claim18, wherein the forming the first particulate sensor further comprisesforming, on the actuation unit, a particulate-adhesion layer.
 21. Themethod according to claim 18, comprising: forming a second buried cavityat the first surface of the wafer and alongside the first buried cavity;forming, on the first surface of the wafer, a first heater elementoverlapping the second buried cavity and configured to generate heatwhen biased; forming an electrically insulating layer on the firstheater element; and forming, on the electrically insulating layer, agas-detection unit, the gas-detection unit including a first plate and asecond plate extending over the electrically insulating layer, the firstplate and the second plate configured to be capacitively coupledtogether.